1. Field of the Invention
The present invention relates to a friction drive actuator suitably used for driving a magnetic recording head of a hard disk drive (hereinafter, also “HDD”) and so-called an ultrasonic actuator, and the hard disk device.
2. Description of the Background Art
An ultrasonic actuator for driving a magnetic recording head of a HDD is required to have a highly accurate positioning performance and a high-speed response.
A typical prior art applicable for such a purpose is disclosed, for example, in Japanese Unexamined Patent Publication No. H06-78570 (hereinafter, “D1”). FIG. 7 is a section of an ultrasonic actuator according to such a prior art. This ultrasonic actuator 100 is a typical example of a general traveling-wave rotary actuator. In this ultrasonic actuator 100, ultrasonic vibration traveling in the circumferential direction of a disk is generated by a piezoelectric element 102 bonded to a disk-shaped vibrator 101 and transmitted to a movable body 104 (rotor) via a liner 103, thereby rotating the movable body 104.
However, in the ultrasonic actuator 100 of this prior art, the movable body 104 is positioned by a bearing 105 comprised of a ball bearing. The ball bearing has backlash between a ball and inner and outer rings although only to a very small extent. This causes displacements of the rotor (movable body 104) and unnecessary resonance. As a result, there is a limit to high accuracy. For example, in the case of the above HDD, there is a limit to the improvement of recording density. In addition to this backlash, the inertial mass of the ball bearing and the bearing load caused by the frictional resistance of the bearing and the viscous resistance of bearing lubricant restrict a response improvement. More specifically, a drive response in an ultrasonic actuator for driving a magnetic recording head of a HDD is normally determined by the resonant frequency of an arm portion, that of a bearing portion, that of a suspension mounted at the leading end, etc. With the miniaturization of the HDD, the resonant frequencies of the arm portion and the suspension can be designed to be relatively high, wherefore the above backlash in the bearing portion, the inertial mass and the bearing load restrict the above response.
For example, a device disclosed in Japanese Unexamined Patent Publication No. 2000-224876 (hereinafter, “D2”) can be cited as another prior art capable of solving such a problem. FIG. 8 is a plan view of an information storage device using a small drive device of this prior art. In this information storage device 110, a cylindrical member 112 is fixed to the base end of a head arm 111 and three or more piezoelectric vibrators 113 are arranged around the member 112. These piezoelectric vibrators 113 undergo elliptical vibration by performing combinations of bending motions and elongating and contracting motions. The member 112 rotates by this elliptical vibration, and the head arm 111 pivots with the member 112 as a rotary shaft. Since the head arm 111 is positioned only by the piezoelectric vibrators 113 in this way without using a bearing such as the above ball bearing, there is no backlash by the bearing and the inertia and load of the bearing are reduced.
However, in the information storage device 110 of this prior art, the head arm 111 is held to be movable in a pressing direction by a spring 114 so that the piezoelectric vibrators 113 for pressing the head arm 111 generate suitable pressing (frictional) forces. Thus, the piezoelectric vibrators 113 are also pivotal in rotating directions of the head arm 111. Accordingly, a drive side also moves and such a movement of the drive side overshoots or undershoots. Therefore, a target position cannot be easily reached, which is a problem in improving the response.
For example, a device disclosed in Japanese Unexamined Patent Publication No. H05-268779 (hereinafter, “D3”) can be cited as still another prior art capable of solving such a problem. FIGS. 9A to 9C are sections of ultrasonic motors according to this prior art. In the ultrasonic motors 120A, 120B and 120C, a stator 122 is formed by providing electrodes on the inner and outer circumferential surfaces of a cylindrical piezoelectric element 121, unevenness formed in the stator 122 in radial directions travels in a circumferential direction to produce a traveling wave, and a rotor 123, 124 or 125 is fitted on the outer circumferential surface of the stator.
In the ultrasonic motors 120A, 120B and 120C of this prior art, a low-rigidity part 123a, 124a or 125a as a part of the rotor 123, 124 or 125 is elastically deformed by an external force to be pressed into contact with the stator 122. Thus, there is no bearing backlash and, hence, no deviation of the center of rotation, and the response is improved.
Generally, in a bearingless friction drive actuator, in which a vibrator directly supports a rotor while being held in contact therewith in a radial direction, effective as a highly responsive rotational driving mechanism, it is important to stabilize a pressing (frictional) force for stabilizing a drive performance. In this respect, in D3, the low-rigidity pars 123a, 124a and 125a of the rotors 123, 124 and 125 are elastically deformed by external forces to generate pressing (frictional) forces to be applied to the stator 122. However, particularly in the constructions shown in FIGS. 9A and 9B, the deformations of cylindrical parts 123b, 124b are actually necessary and the deformations of the constricted low-rigidity parts 123a, 124a hardly contribute to the above pressing forces. In addition, the cylindrical parts 123b, 124b need to be deformed without being flattened while being kept in cylindrical shapes (to have arcuate cross sections in an axial direction like the low-rigidity part 125a of FIG. 9C without being squeezed). However, the low-rigidity parts 123a, 124a are actually not deformed well in such a manner. Therefore, there is a problem of being unable to stabilize the pressing (frictional) forces.
In the construction of FIG. 9C as well, the low-rigidity part 125a has a low rigidity since being a thin plane. However, since the low-rigidity part needs to be deformed without being flattened as described above, the actual solid shape is not low in rigidity. In other words, if a part is cut out in a circumferential direction, the low-rigidity part is easily deformable in the above arcuate shape. However, if the low-rigidity part has a constant thickness over the entire circumference, the rigidity thereof is far from low. The bearing backlash can be eliminated if such a large external force as to exceed the rigidity and elastically deform the low-rigidity part is applied. However, the pressing (frictional) force becomes unstable in a method accompanied by the deformation of such a high-rigidity part, thereby presenting a problem of making the drive performance unstable. Further, if the external force is large, there is also a problem that a mechanism for applying it tends to be enlarged.